First you need 2 tools:

- KVS2OGG for conversion (available on http://dl.deiyu-studio.net/kvs2ogg/)

- Any hex editing program of your preference (in this tutorial I'm using HxD)

In addition you need to have a BGM file that is in .mp3 or .wav format

(skip this step and 1a if you're familiar with changing BGM file format)

Prepare the custom BGM file with proper format mentioned above, then open the file in KVS2OGG and convert it to .kvs using the "Tool" drop-down tab at the top

Alternatively if you want your custom BGM to play with a loop timestamp, convert it to .ogg first before .kvs

You'll be given the option to set at which second the BGM will loop

Finally don't forget to change the filename extension from .kvs to .g1l before proceeding to next step

This part is a little tricky and could damage the file intended to be used on your game so make a backup of it first.

Locate the game BGM files in the game directory (....\Steam\steamapps\common\Romance of the Three Kingdoms 13\bgm) and copy the BGM you want to replace (don't forget to backup)
For example say you want to replace the main menu BGM, copy the BGM01.g1l to your work folder and open it in hex editor (along with your custom BGM .g1l file)
rename the original BGM01 file into something like "BGM01ori.g1l" so it won't overlap

Copy the first 28 bytes of BGM01.g1l file (every line before 'KOVS' on the ASCII table) and paste it onto the custom BGM file in the similar postion (0x00)*

Careful as copying wrong amount of bytes will brick your BGM file

Save and rename the custom BGM file to "BGM01.g1l" and move it back to the game BGM folder. Start the game to see the result

* = It is recommended that you use the exact copy of the BGM you want to replace to edit the custom BGM. For example if you want to
swap BGM02 then you have to copy the bytes from BGM02 to your custom BGM

These are what I've identified so far.

BGM01 - Invitation to the Three Kingdoms
BGM02 - Cao Cao's Theme
BGM03 - Liu Bei's Theme
BGM04 - Sun Quan's Theme
BGM05 - Zhuge Liang's Theme
BGM06 - Zhao Yun's Theme
BGM07 - Guan Yu's Theme
BGM08 - Zhou Yu's Theme
BGM09 - Lu Bu's Theme
BGM10 - Woman's Theme
BGM11 - Theme of the North
BGM12 - Theme of the Central Plains
BGM13 - Theme of the Northwest
BGM14 - Theme of Wuyue
BGM15 - Theme of Jingchu
BGM16 - Theme of Bashu
BGM17 - Emperor's Theme
BGM18 - Theme of Chaisang
BGM19 - Marching
BGM20 - Defense
BGM21 - Normal Battle
BGM22 - Winning Battle
BGM23 - Losing Battle
BGM24 - City/Checkpoint Battle
BGM25 - Winning City/Checkpoint Battle
BGM26 - Losing City/Checkpoint Battle
BGM27 - Naval Battle
BGM28 - Winning Naval Battle
BGM29 - Losing Naval Battle
BGM30 - After the Battle
BGM31 - Duel
BGM32 - Debate
BGM33 - Town Hall
BGM34 - Battlefield
BGM35 - Daring
BGM36 - Danger
BGM37 - Conspiracy
BGM38 - Tragedy
BGM39 - Collision
BGM40 - Plotting
BGM41 - Success
BGM42 - Peace
BGM43 - Towards a New Age
BGM52 - ?????
BGM54 - ?????
BGM55 - ?????
BGM56 - ?????
BGM57 - ?????
BGM58 - ?????
BGM59 - ?????
BGM60 - ?????
BGM61 - ?????
BGM62 - ?????
BGM63 - ?????
BGM64 - ?????
BGM65 - ?????
BGM66 - Prestige
BGM67 - Prestige/Pomp
BGM68 - War Council
BGM69 - Main Camp
BGM70 - Edit/Collection
BGM71 - Family Theme
BGM72 - Sima Yi's Theme
BGM73 - Zhang Fei's Theme
BGM74 - Nanman's Theme
BGM75 - ?????
BGM76 - ?????

I started playing in anticipation of XIV (which certainly will take several months later to be released in english, not to mention the PUK and other extras) hopefully some who are still playing this game will find my guide helpful

The looping point can sometimes be inaccurate as KVS2OGG can only set timestamp to seconds scale. You need media editor to set the loop timestamp accurate to less than a second (I used Premier Pro personally)